Secretary of State Hillary Rodham Clinton says Russia is sending attack helicopters to Syria. A U.N. official says the conflict has become a civil war.

WASHINGTON — Secretary of State Hillary Rodham Clinton charged Tuesday that Russia is sending helicopter gunships to prop up the Syrian government in an escalating conflict with rebels that a top U.N. official acknowledged had become a full-scale civil war.

Russia has denied that it is supplying Syrian President Bashar Assad's security forces with weapons that can be used against armed rebels and civilians. Heavy fighting has claimed at least 10,000 lives over 15 months.

"We have confronted the Russians about stopping their continued arms shipments to Syria," Clinton said at the Brookings Institution think tank. "They have, from time to time, said that we shouldn't worry, everything they are shipping is unrelated to [the Syrian government's] actions internally. That's patently untrue.

"And we are concerned about the latest information we have that there are attack helicopters on the way from Russia to Syria, which will escalate the conflict quite dramatically," she said.

Victoria Nuland, the State Department spokeswoman, later declined to disclose the source of the information.

Syrian forces appear to have launched an offensive in recent days to regain areas, towns and neighborhoods lost to rebels, including the central province of Homs, Idlib province in the northwest, coastal Latakia province, the eastern city of Dair Alzour and the restive suburbs of Aleppo, Syria's business hub.

Even as diplomats scramble to revive U.N. special envoy Kofi Annan's six-point peace plan, the Syrian government seems to have dropped any pretense of abiding by several of its core provisions: those mandating a government drawdown of troops and armor from populated zones.

U.N. monitors in Syria have in recent days documented the use of attack helicopters, artillery and tanks.

"I think there is a massive increase in the level of violence, so massive indeed that in a way it indicates some change of nature," French diplomat Herve Ladsous, head of U.N. peacekeeping, told a small group of reporters. "This is becoming large-scale, because the opposition also resists."

Asked whether the country had slid into a full-fledged civil war, the outcome that the U.N. has been trying to avoid, he answered, "Yes, I think we can say that," according to the Agence France-Presse news agency. "Clearly what is happening is that the government of Syria has lost some large chunks of territory, several cities to the opposition, and wants to retake control."

Although various world leaders have voiced fear that Syria was slipping into a civil war, Ladsous appeared to be the most senior U.N. official to publicly declare that it had begun.

Ladsous made his comments on a day when an angry crowd, apparently made up of government supporters, threw rocks and metal rods at a U.N. convoy trying to reach the besieged town of Haffah in the foothills east of Latakia. The U.S. State Department has expressed concern that the Syrian government could be planning a massacre there.

The convoy was forced to turn around, and unknown assailants later opened fire at three U.N. vehicles, the United Nations said. No one was hurt in either incident, it said.

In another sign of tension between Syria and U.N. observers, the official Syrian news agency reported that a U.N. observer's car ran over three people during the visit to Latakia, leaving two of them in critical condition.

Rapidly escalating violence raised questions about the future of the U.N. observer mission, whose mandate expires in late July.

"If there is no discernible movement by then, it will be very difficult to extend a mission that is increasingly dangerous for the observers on the ground," Clinton said.

A Pentagon spokesman, Navy Capt. John Kirby, said he could not confirm Clinton's allegation that Russia was supplying attack helicopters, but he accused the Assad regime of repeatedly using military helicopters against civilians.

Clinton's charge is likely to sharpen friction between the United States and Russia at a time when the Obama administration is seeking help from Moscow on several fronts.

The next round of international talks on Iran'sdisputed nuclear program is scheduled to begin Monday in Moscow, and U.S. and European diplomats are looking to Russia to use its influence with the Iranians during that session.

President Obama is expected to meet in private with Russian President Vladimir Putin on the sidelines of the summit of the Group of 20 industrial nations, which also begins Monday, in Los Cabos, Mexico.